Move documentation files from egcs/gcc/objc.
From-SVN: r22461
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+This file describes in little detail the modifications to the
+Objective-C runtime needed to make it thread safe.
+
+First off, kudos to Galen Hunt who is the author of this great work.
+
+If you have an comments or just want to know where to
+send me money to express your undying gratitude for threading the
+Objective-C runtime you can reach Galen at:
+
+ gchunt@cs.rochester.edu
+
+Any questions, comments, bug reports, etc. should send email either to the
+GCC bug account or to:
+
+ Scott Christley <scottc@net-community.com>
+
+* Sarray Threading:
+
+The most critical component of the Objective-C runtime is the sparse array
+structure (sarray). Sarrays store object selectors and implementations.
+Following in the tradition of the Objective-C runtime, my threading
+support assumes that fast message dispatching is far more important
+than *ANY* and *ALL* other operations. The message dispatching thus
+uses *NO* locks on any kind. In fact, if you look in sarray.h, you
+will notice that the message dispatching has not been modified.
+Instead, I have modified the sarray management functions so that all
+updates to the sarray data structure can be made in parallel will
+message dispatching.
+
+To support concurrent message dispatching, no dynamically allocated
+sarray data structures are freed while more than one thread is
+operational. Sarray data structures that are no longer in use are
+kept in a linked list of garbage and are released whenever the program
+is operating with a single thread. The programmer can also flush the
+garbage list by calling sarray_remove_garbage when the programmer can
+ensure that no message dispatching is taking place concurrently. The
+amount of un-reclaimed sarray garbage should normally be extremely
+small in a real program as sarray structures are freed only when using
+the "poseAs" functionality and early in program initialization, which
+normally occurs while the program is single threaded.
+
+******************************************************************************
+* Static Variables:
+
+The following variables are either statically or globally defined. This list
+does not include variables which are internal to implementation dependent
+versions of thread-*.c.
+
+The following threading designations are used:
+ SAFE : Implicitly thread safe.
+ SINGLE : Must only be used in single thread mode.
+ MUTEX : Protected by single global mutex objc_runtime_mutex.
+ UNUSED : Not used in the runtime.
+
+Variable Name: Usage: Defined: Also used in:
+=========================== ====== ============ =====================
+__objc_class_hash MUTEX class.c
+__objc_class_links_resolved UNUSED class.c runtime.h
+__objc_class_number MUTEX class.c
+__objc_dangling_categories UNUSED init.c
+__objc_module_list MUTEX init.c
+__objc_selector_array MUTEX selector.c
+__objc_selector_hash MUTEX selector.c
+__objc_selector_max_index MUTEX selector.c sendmsg.c runtime.h
+__objc_selector_names MUTEX selector.c
+__objc_thread_exit_status SAFE thread.c
+__objc_uninstalled_dtable MUTEX sendmsg.c selector.c
+_objc_load_callback SAFE init.c objc-api.h
+_objc_lookup_class SAFE class.c objc-api.h
+_objc_object_alloc SINGLE objects.c objc-api.h
+_objc_object_copy SINGLE objects.c objc-api.h
+_objc_object_dispose SINGLE objects.c objc-api.h
+frwd_sel SAFE2 sendmsg.c
+idxsize MUTEX sarray.c sendmsg.c sarray.h
+initialize_sel SAFE2 sendmsg.c
+narrays MUTEX sarray.c sendmsg.c sarray.h
+nbuckets MUTEX sarray.c sendmsg.c sarray.h
+nindices MUTEX sarray.c sarray.h
+previous_constructors SAFE1 init.c
+proto_class SAFE1 init.c
+unclaimed_categories MUTEX init.c
+unclaimed_proto_list MUTEX init.c
+uninitialized_statics MUTEX init.c
+
+Notes:
+1) Initialized once in unithread mode.
+2) Initialized value will always be same, guaranteed by lock on selector
+ hash table.
+
+
+******************************************************************************
+* Frontend/Backend design:
+
+The design of the Objective-C runtime thread and mutex functions utilizes a
+frontend/backend implementation.
+
+The frontend, as characterized by the files thr.h and thr.c, is a set
+of platform independent structures and functions which represent the
+user interface. Objective-C programs should use these structures and
+functions for their thread and mutex work if they wish to maintain a
+high degree of portability across platforms.
+
+The backend is composed of a file with the necessary code to map the ObjC
+thread and mutex to a platform specific implementation. For example, the
+file thr-solaris.c contains the implementation for Solaris. When you
+configure GCC, it attempts to pick an appropriate backend file for the
+target platform; however, you can override this choice by assign the
+OBJC_THREAD_FILE make variable to the basename of the backend file. This
+is especially useful on platforms which have multiple thread libraries.
+For example:
+
+ make OBJC_THREAD_FILE=thr-posix
+
+would indicate that the generic posix backend file, thr-posix.c, should be
+compiled with the ObjC runtime library. If your platform does not support
+threads then you should specify the OBJC_THREAD_FILE=thr-single backend file
+to compile the ObjC runtime library without thread or mutex support; note
+that programs which rely upon the ObjC thread and mutex functions will
+compile and link correctly but attempting to create a thread or mutex will
+result in an error.
+
+It is questionable whether it is really necessary to have both a
+frontend and backend function for all available functionality. On the
+one hand, it provides a clear, consistent differentiation between what
+is public and what is private with the downside of having the overhead
+of multiple functions calls. For example, the function to have a thread
+yield the processor is objc_thread_yield; in the current implementation
+this produces a function call set:
+
+objc_thread_yield() -> __objc_thread_yield() -> system yield function
+
+This has two extra function calls over calling the platform specific function
+explicitly, but the issue is whether only the overhead of a single function
+is necessary.
+
+objc_thread_yield() -> system yield function
+
+This breaks the public/private dichotomy between the frontend/backend
+for the sake of efficiency. It is possible to just use a preprocessor
+define so as to eliminate the extra function call:
+
+#define objc_thread_yield() __objc_thread_yield()
+
+This has the undesirable effect that if objc_thread_yield is actually
+turned into a function based upon future need; then ObjC programs which
+access the thread functions would need to be recompiled versus just
+being relinked.
+
+******************************************************************************
+* Threads:
+
+The thread system attempts to create multiple threads using whatever
+operating system or library thread support is available. It does
+assume that all system functions are thread safe. Notably this means
+that the system implementation of malloc and free must be thread safe.
+If a system has multiple processors, the threads are configured for
+full parallel processing.
+
+* Backend initialization functions
+
+__objc_init_thread_system(void), int
+ Initialize the thread subsystem. Called once by __objc_exec_class.
+ Return -1 if error otherwise return 0.
+
+__objc_close_thread_system(void), int
+ Closes the thread subsystem, not currently guaranteed to be called.
+ Return -1 if error otherwise return 0.
+
+*****
+* Frontend thread functions
+* User programs should use these functions.
+
+objc_thread_detach(SEL selector, id object, id argument), objc_thread_t
+ Creates and detaches a new thread. The new thread starts by
+ sending the given selector with a single argument to the
+ given object.
+
+objc_thread_set_priority(int priority), int
+ Sets a thread's relative priority within the program. Valid
+ options are:
+
+ OBJC_THREAD_INTERACTIVE_PRIORITY
+ OBJC_THREAD_BACKGROUND_PRIORITY
+ OBJC_THREAD_LOW_PRIORITY
+
+objc_thread_get_priority(void), int
+ Query a thread's priority.
+
+objc_thread_yield(void), void
+ Yields processor to another thread with equal or higher
+ priority. It is up to the system scheduler to determine if
+ the processor is taken or not.
+
+objc_thread_exit(void), int
+ Terminates a thread. If this is the last thread executing
+ then the program will terminate.
+
+objc_thread_id(void), int
+ Returns the current thread's id.
+
+objc_thread_set_data(void *value), int
+ Set a pointer to the thread's local storage. Local storage is
+ thread specific.
+
+objc_thread_get_data(void), void *
+ Returns the pointer to the thread's local storage.
+
+*****
+* Backend thread functions
+* User programs should *NOT* directly call these functions.
+
+__objc_thread_detach(void (*func)(void *arg), void *arg), objc_thread_t
+ Spawns a new thread executing func, called by objc_thread_detach.
+ Return NULL if error otherwise return thread id.
+
+__objc_thread_set_priority(int priority), int
+ Set the thread's priority, called by objc_thread_set_priority.
+ Return -1 if error otherwise return 0.
+
+__objc_thread_get_priority(void), int
+ Query a thread's priority, called by objc_thread_get_priority.
+ Return -1 if error otherwise return the priority.
+
+__objc_thread_yield(void), void
+ Yields the processor, called by objc_thread_yield.
+
+__objc_thread_exit(void), int
+ Terminates the thread, called by objc_thread_exit.
+ Return -1 if error otherwise function does not return.
+
+__objc_thread_id(void), objc_thread_t
+ Returns the current thread's id, called by objc_thread_id.
+ Return -1 if error otherwise return thread id.
+
+__objc_thread_set_data(void *value), int
+ Set pointer for thread local storage, called by objc_thread_set_data.
+ Returns -1 if error otherwise return 0.
+
+__objc_thread_get_data(void), void *
+ Returns the pointer to the thread's local storage.
+ Returns NULL if error, called by objc_thread_get_data.
+
+
+******************************************************************************
+* Mutexes:
+
+Mutexes can be locked recursively. Each locked mutex remembers
+its owner (by thread id) and how many times it has been locked. The
+last unlock on a mutex removes the system lock and allows other
+threads to access the mutex.
+
+*****
+* Frontend mutex functions
+* User programs should use these functions.
+
+objc_mutex_allocate(void), objc_mutex_t
+ Allocates a new mutex. Mutex is initially unlocked.
+ Return NULL if error otherwise return mutex pointer.
+
+objc_mutex_deallocate(objc_mutex_t mutex), int
+ Free a mutex. Before freeing the mutex, makes sure that no
+ one else is using it.
+ Return -1 if error otherwise return 0.
+
+objc_mutex_lock(objc_mutex_t mutex), int
+ Locks a mutex. As mentioned earlier, the same thread may call
+ this routine repeatedly.
+ Return -1 if error otherwise return 0.
+
+objc_mutex_trylock(objc_mutex_t mutex), int
+ Attempts to lock a mutex. If lock on mutex can be acquired
+ then function operates exactly as objc_mutex_lock.
+ Return -1 if failed to acquire lock otherwise return 0.
+
+objc_mutex_unlock(objc_mutex_t mutex), int
+ Unlocks the mutex by one level. Other threads may not acquire
+ the mutex until this thread has released all locks on it.
+ Return -1 if error otherwise return 0.
+
+*****
+* Backend mutex functions
+* User programs should *NOT* directly call these functions.
+
+__objc_mutex_allocate(objc_mutex_t mutex), int
+ Allocates a new mutex, called by objc_mutex_allocate.
+ Return -1 if error otherwise return 0.
+
+__objc_mutex_deallocate(objc_mutex_t mutex), int
+ Free a mutex, called by objc_mutex_deallocate.
+ Return -1 if error otherwise return 0.
+
+__objc_mutex_lock(objc_mutex_t mutex), int
+ Locks a mutex, called by objc_mutex_lock.
+ Return -1 if error otherwise return 0.
+
+__objc_mutex_trylock(objc_mutex_t mutex), int
+ Attempts to lock a mutex, called by objc_mutex_trylock.
+ Return -1 if failed to acquire lock or error otherwise return 0.
+
+__objc_mutex_unlock(objc_mutex_t mutex), int
+ Unlocks the mutex, called by objc_mutex_unlock.
+ Return -1 if error otherwise return 0.
+
+******************************************************************************
+* Condition Mutexes:
+
+Mutexes can be locked recursively. Each locked mutex remembers
+its owner (by thread id) and how many times it has been locked. The
+last unlock on a mutex removes the system lock and allows other
+threads to access the mutex.
+
+*
+* Frontend condition mutex functions
+* User programs should use these functions.
+*
+
+objc_condition_allocate(void), objc_condition_t
+ Allocate a condition mutex.
+ Return NULL if error otherwise return condition pointer.
+
+objc_condition_deallocate(objc_condition_t condition), int
+ Deallocate a condition. Note that this includes an implicit
+ condition_broadcast to insure that waiting threads have the
+ opportunity to wake. It is legal to dealloc a condition only
+ if no other thread is/will be using it. Does NOT check for
+ other threads waiting but just wakes them up.
+ Return -1 if error otherwise return 0.
+
+objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int
+ Wait on the condition unlocking the mutex until objc_condition_signal()
+ or objc_condition_broadcast() are called for the same condition. The
+ given mutex *must* have the depth 1 so that it can be unlocked
+ here, for someone else can lock it and signal/broadcast the condition.
+ The mutex is used to lock access to the shared data that make up the
+ "condition" predicate.
+ Return -1 if error otherwise return 0.
+
+objc_condition_broadcast(objc_condition_t condition), int
+ Wake up all threads waiting on this condition. It is recommended that
+ the called would lock the same mutex as the threads in
+ objc_condition_wait before changing the "condition predicate"
+ and make this call and unlock it right away after this call.
+ Return -1 if error otherwise return 0.
+
+objc_condition_signal(objc_condition_t condition), int
+ Wake up one thread waiting on this condition.
+ Return -1 if error otherwise return 0.
+
+*
+* Backend condition mutex functions
+* User programs should *NOT* directly call these functions.
+*
+
+__objc_condition_allocate(objc_condition_t condition), int
+ Allocate a condition mutex, called by objc_condition_allocate.
+ Return -1 if error otherwise return 0.
+
+__objc_condition_deallocate(objc_condition_t condition), int
+ Deallocate a condition, called by objc_condition_deallocate.
+ Return -1 if error otherwise return 0.
+
+__objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int
+ Wait on the condition, called by objc_condition_wait.
+ Return -1 if error otherwise return 0 when condition is met.
+
+__objc_condition_broadcast(objc_condition_t condition), int
+ Wake up all threads waiting on this condition.
+ Called by objc_condition_broadcast.
+ Return -1 if error otherwise return 0.
+
+__objc_condition_signal(objc_condition_t condition), int
+ Wake up one thread waiting on this condition.
+ Called by objc_condition_signal.
+ Return -1 if error otherwise return 0.